The standard method for collecting field data of sediments/sedimentary rocks is to construct a graphic log of the succession. Logs immediately give a visual impression of the section, and are a convenient way of making correlations and comparisons between equivalent sections from different areas. Repetitions of facies, sedimentary cycles and general trends may become apparent, such as a systematic upward change in bed or cycle thickness or in grain-size, increasing or decreasing upward. In addition, the visual display of a graphic log helps with the interpretation of the succession.
However, a log does emphasise the vertical changes in the succession, at the expense of lateral variations. The vertical scale to use depends on the detail required, sediment variability, and time available. For precise work on short sections, 1:10 or 1:5 is used, but for many purposes 1:50 (that is, 1 cm on the log equals 0.5 m) or 1:100 (1 cm to 1 m) is adequate. In some situations, it may not be necessary to log the whole succession, or to log the whole succession at the same scale. A representative log may be sufficient.
There is no set format for a graphic log; indeed, the features which can be recorded vary from succession to succession. Features which it is necessary to record and which therefore require a column on the log are bed or rock unit thickness, lithology, texture (especially grain-size), sedimentary structures, palaeocurrents, colour and fossils. The nature of bed contacts can also be marked on the log.
If you are going to spend some time in the field then it is worth preparing the log sheets before you go. An alternative is to construct a log in your field notebook, but this is usually less satisfactory since the page size of most notebooks is too small.
Where the exposure is continuous or nearly so, there is no problem concerning the line of the log; simply take the easiest path. If the outcrop is good but not everywhere continuous it may be necessary to move laterally along the section to find outcrops of the succeeding beds. Some small excavations may be required where rocks in the succession, commonly mudrocks, are not exposed; otherwise enter ‘no exposure’ on the log. It is best to log from the base of the succession upwards. In this way you are recording how deposition changed as time progressed, rather than back through time, and it is generally easier to identify bed boundaries and facies changes by moving up the section.
Bed or rock-unit thickness
The thickness is measured with a tape measure; care must be exercised where rocks dip at a high angle and the exposure surface is oblique to the bedding. Attention needs to be given to where boundaries are drawn between units in the succession; if there are obvious bedding planes or changes in lithology then there is no problem. Thin beds, all appearing identical, can be grouped together into a single lithological unit on the log, if a large scale is being used. Where there is a rapid alternation of thin beds of different lithology, e.g. interbedded sandstones and shales (heterolithics), they can be treated as one unit and notes made of the thicknesses and character of individual beds, noting any increases or decreases in bed thickness up through the unit.
Thus, when first approaching a section for logging, stand back a little and see where the natural breaks come in the succession to define the various beds or rock units.
On the graphic log, lithology is recorded in a column by using an appropriate ornamentation; see Fig. If it is possible to subdivide the lithologies further, then more symbols can be added, or coloured pencils used. If two lithologies are thinly interbedded, then the column can be divided in two by a vertical line and the two types of ornament entered. More detailed comments and observations on the lithology should be entered in the field notebook, reference to the bed or rock unit being made by its number.
On the log there should be a horizontal scale for the textural column. For many rocks this will show mud (clay + silt), sand (divided into fine, medium and coarse) and gravel. Gravel can be divided further if coarse sediments are being logged. To aid the recording of grain-size (or crystal-size), fine vertical lines can be drawn for each grain-size class boundary. Having determined the grain-size of a rock unit, mark this on the log and shade the area; the
wider the column, the coarser the rock. Ornament for the lithology and/or sedimentary structures can be added to this textural column. In many logs, lithology and texture are combined into one column.
Other textural features, such as grain fabric, roundness and shape, should be recorded in the field notebook, although distinctive points can be noted in the remarks column. Particular attention should be given to these features if conglomerates and breccias are in the succession.
For the graphic logging of carbonate rocks, it is useful to combine the lithology/texture columns and use the Dunham classification. Thus you could have columns for lime mudstone (M), wackestone (W), packstone (P) and grainstone (G); a column for boundstones (B) can be added if reef-rocks or stromatolites are present. If there are very coarse limestones, separate columns can be added for rudstones (R) and floatstones (F)
Sedimentary structures and bed contacts
Sedimentary structures and bed contacts within the strata can be recorded in a column by symbols. Sedimentary structures occur on the upper and lower surfaces of beds as well as within them. Separate columns could be used for surface and internal sedimentary structures if they are both common. Symbols for the common sedimentary structures are shown in Fig.. Measurements, sketches and descriptions of the structures should be made in the field notebook.
Note whether bed boundaries are (a) sharp and planar, (b) sharp and scoured, or (c) gradational; each can be represented in the lithology column by a straight, wavy/irregular or dashed line respectively.
For the graphic log, readings can be entered either in a separate column or adjacent to the textural log as an arrow or trend line. The measurements themselves should be retained in the field notebook; make a table for the readings.
Fossils indicated on the graphic log should record the principal fossil groups present in the rocks. Symbols which are commonly used are shown in Fig. These can be placed in a fossil column alongside the sedimentary structures.
If fossils make up much of the rock (as in some limestones) then the symbol(s) of the main group(s) can be used in the lithology column. Separate subcolumns on the textural log could be designated for rudstones and floatstones, where large fossils are abundant and in contact or in matrix–support fabric respectively.
The colour of a sedimentary rock is best recorded by use of a colour chart, but if this is not available then simply devise abbreviations for the colour column.
This can be used for special features of the bed or rock unit, such as degree of weathering (see Section 4.7) and presence of authigenic minerals (pyrite, glauconite, etc.), and for supplementary data on the sedimentary structures, texture or lithology. The presence of joints and fractures can also be noted here (their spacing and density: see Section 5.5.8). Specimen numbers can be entered here, as well as the location of photographs taken, and crossreferences to sketches in your notebook.